a(n) = 3*a(n-1) + 18*a(n-2), a(0)=0, a(1)=1.

Terms

a(0) =0a(1) =1a(2) =3a(3) =27a(4) =135a(5) =891a(6) =5103a(7) =31347a(8) =185895a(9) =1121931a(10) =6711903a(11) =40330467a(12) =241805655a(13) =1451365371a(14) =8706597903a(15) =52244370387a(16) =313451873415